Microencapsulation of reactive formulation components can be chemically achieved through single or multiple depositions of shell wall materials to core materials. Such processes include Interfacial Polymerization, Complex Coacervation and In-Situ Polymerization. These processes can be used individually to deposit a single shell wall over a desired core material.
In addition, there are instances in which multiple layers of a single type of shell wall or multiple layers of various combinations of shell wall materials are used to achieve certain microcapsule properties. These properties include minimization of “free” core material or material that does not get encapsulated, increased shelf-life of the finished microcapsules, increased protection of the core material from environmental conditions imposed upon the microcapsules due to incorporation into a formulation involving either polar or non-polar solvents.
However, performing multi-layer shell deposition and/or multiple-process microencapsulation complicates and can greatly increase the number of steps involved in forming microcapsules of the desired quality. Thus, single shell deposition frequently is utilized to address these issues.
Further, complications in microencapsulation are more pronounced with certain compounds. Such is the case with the blowing agent methyl hydrogen silicone fluid. Because of the reactivity of this core material, the chemical microencapsulation processes available for use are limited to Complex Coacervation and In-Situ Polymerization. However, single-shell microencapsulation by either of these methods individually usually is not adequate to fully encapsulate the blowing agent, resulting in unencapsulated core material, poorly formed caps or shells, leaky caps or shells that enable the core material to escape the shell, or no caps or shells, depending on the formation conditions.
A combination of these two methods, Complex Coacervation and In-Situ Polymerization, can be used to deposit first a gelatin shell via Complex Coacervation followed by deposition of a polyoxymethylene urea shell via In-Situ Polymerization, or these processes can be used in reverse to deposit first a polyoxymethylene urea via In-Situ Polymerization followed by a gelatin shell via Complex Coacervation. While this provides slightly better results than the single-shell process, in that microcapsule formation is more often achieved, the microcapsules formed typically are not in the form of dry, free-flowing powder, and still tend to leak or have large amounts of “free” core material that remains un-encapsulated. Thus, microcapsules so formed often are not adequate for use in a final formulation because they are difficult to handle and process, and when mixed with other reactive components, the core material tends to immediately react with those components, which results in early expansion of the material before expiration of a desired latency period. Further, in cases where the inferior microcapsules are sufficiently formed, they typically do not provide the desired material shelf-life.